All runtime-allocated memory in a Rust program begins its life as uninitialized. In this state the value of the memory is an indeterminate pile of bits that may or may not even reflect a valid state for the type that is supposed to inhabit that location of memory. Attempting to interpret this memory as a value of any type will cause Undefined Behavior.
That's not true; there are types that allow reading a value from uninitialized memory for some or all of their bytes: MaybeUninit is the obvious example, and more generally unions with a () field (or even all unions, details are TBD). Also, padding bytes are allowed to be uninitialized.
Here, the Nomicon claims
That's not true; there are types that allow reading a value from uninitialized memory for some or all of their bytes:
MaybeUninitis the obvious example, and more generally unions with a()field (or even all unions, details are TBD). Also, padding bytes are allowed to be uninitialized.